After all of the work and effort that her supporters and followers invested in the last two years in preparation for the swim, Nyad said it was unlikely that she would attempt the Cuba to Florida swim again.

After much consideration, Nyad made the decision to call off the swim and was quickly taken aboard the Bellissimo, the 75-foot yacht that carried her handlers, drivers, photographers, as well as friends and family.

Once on the boat, under her own power, Nyad worked her way up the stairs and into a waiting chair, overwhelmed with exhaustion and emotion. She was given intravenous fluids as she battled nausea.

Some three hours later, she rose to her feet and walked inside the boat. There, lying down, she talked to those on board, a mini-press conference of sorts with half a dozen cameras at work.

Nyad said that as early as the third hour of her journey she began experiencing pain in her right shoulder. By hour 15, asthma was a problem.

As hour 28 approached, the pain was so great that she had to rest every 30 to 40 freestyle strokes, rolling onto her back to breathe.

Her doctor, Michael Broder, would swim to her on breaks to monitor vitals or administer medication.

"I'm hurting, I'm hurting," Nyad told him, clutching her shoulder and looking to the stars. Then, she'd turn back into the water, struggling through another stroke or two, pushing and pushing and pushing.

By hour 29, she was weighing the math between belabored strokes.

She asked: "Do I have to swim all night and all day and all night again?" The answer was yes.

Nyad switched to a breast stroke, a last hope to find a way to propel herself to land. A handful of strokes. More rest. A few minutes more. And, finally, she made the decision. It was over.

"Asthma took so much out of me," she told CNN Chief Medical Correspondent Sanjay Gupta. "I couldn't overcome it."

Nyad said she took at least 40 inhaler puffs and even took oxygen to help improve her breathing.

"We did everything we could to get out of asthma distress, but I couldn't have gone another hour, couldn't have done it," she said.

"I really believed willpower would get me through, but the asthma really shafted me," she added.

She willed herself to go as far as she did, and was willing to doggy-paddle to the end if necessary, she told CNN. After the seas kicked up and she was ailing, however, she knew mental toughness alone would not be enough.

"I just knew it wasn't mind over matter anymore. I was absolutely spent," she said.

After her rigorous training -- which included a 24-hour swim for practice, ending her swim was "a bitter pill to swallow," Nyad said.

It was an end to a journey that started off looking much brighter.

"Earlier in the evening, she was surrounded by dolphins and a beautiful Caribbean sunset. But strong currents blew her 15 miles off course," her team posted on her Twitter account.

The attempt to swim from Cuba to Florida was the second for the swimmer, who said at a news conference Sunday that she is fitter today than she was in 1978, when she first attempted the crossing but was unable to finish.

It took several months to gain permission for the swim from Cuban and U.S. authorities. Bureaucratic snags repeatedly threatened to call off the effort -- already called off in 2010 because of weather.

"To swim between these two neighbors, Cuba and the United States, who've been strangers all these years, is a moving thing for me," Nyad had said.

She had been training for the event for two years, swimming up to 12 hours a day.

A team of more than 30 people supported Nyad as she attempted the crossing. She had 10 handlers to advise her as she swam, ocean kayakers towing devices to repel most sharks and divers and safety officers trained to distract sharks that were not turned away.

The lesson that others can take away from her journey is to be your best self, she said.

"You have to live your life with passion, you show your will, you feel proud of yourself when you go to bed at night," she said.